Conductive plastic molding material, the use thereof and moulded bodies produced therefrom

ABSTRACT

The present invention relates to a plastics molding composition based on polyarylene sulfide and/or on liquid-crystalline plastic, where the molding composition comprises carbon black and graphite and/or metal powder, the carbon black has a specific surface area of from 500 to 1500 m 2 /g, and a dibutyl phthalate value of from 100 to 700 ml/100 g, and the graphite has a specific surface area of from 1 to 35 m 2 /g. The molding compositions of the invention have good conductivities, and better flowabilities and mechanical properties.

DESCRIPTION

The present invention relates to conductive plastics molding compositions, in particular high-conductivity plastics molding compositions with intrinsic resistivity RD of less than 10 Ωcm, using liquid-crystalline plastic or polyphenylene sulfide as matrix polymers. Since there is increasingly a shortage of non-renewable energy sources, there is interest in research into fuel cells. Until now, one of the most costly parts of a fuel cell has been the bipolar plate.

Currently, V2A and V4A steels are used to produce the bipolar plates. The disadvantages here are high material costs, difficulties in working the material, corrosion resistance, which is frequently inadequate, and also high weight, since densities are in the range 7-8 g/ml.

Although aluminum is lighter, with densities in the region of 2.7 g/ml, it is likewise costly and forms oxide layers which increase surface resistance.

The density of graphite is 2.24 g/ml, but its mechanical stability is low, and it is therefore impossible to achieve the desired reduction in thickness. In addition, other problems arise due to gas permeation into the layer lattice structure.

Graphite/polymer composites based on thermoset resin, for example, have been developed in order to overcome these disadvantages. U.S. Pat. No. 4,339,322 discloses compounds based on fluorinated and partially fluorinated polymers. However, disadvantages here are the lack of recyclability, and high cycle times in production.

It is known that the structure of carbon black/polymer compounds, even at from 5 to 20 percent by weight carbon black content, gives them boundary conductivities of about 10 S/cm. However, a disadvantage of these molding compositions is their poor flowability, which adversely affects processability.

WO 00/30202 describes a carbon-containing compound based on polyphenylene sulfide or on liquid-crystalline plastic. Here, carbon powder and carbon fibers are used in combination.

It is an object of the present invention to use simple measures to overcome the disadvantages of the prior art. This object is achieved by means of a plastics molding composition based on polyarylene sulfide and/or on liquid-crystalline plastic, where the compound comprises carbon black and graphite and/or metal powder, the carbon black has a specific surface area of from 500 to 1500 m²/g, and a dibutyl phthalate value of from 100 to 700 ml/100 g, and the graphite has a specific surface area of from 1 to 35 m²/g.

The filler contents φM by weight of the molding compositions of the invention are non-zero and less than 85, preferably less than or equal to 80, and particularly preferably in the range from 60 to 80.

The molding compositions of the invention may advantageously also comprise lubricants with internal or external lubricant action, and these may also be removed after the compounding process.

Surprisingly, it has been found that an unexpected synergistic effect occurs in the molding composition of the invention, due to the use of carbon black together with graphite and/or metal powder.

Compared with conventional carbon black compounds, the molding compositions of the invention have better electrical conductivities and thermal conductivities, together with improved flowability and improved mechanical properties. Compared with graphite compounds, the molding compositions of the invention have the same electrical conductivity and similar thermal conductivity, together with reduced density and higher strength.

The carbon black used may be a conductivity black with a specific surface area of from 500 to 1500 m²/g, advantageously from 800 to 1250 m²/g. The carbon blacks suitable according to the invention moreover have a dibutyl phthalate value of from 100 to 700 ml/100 g, advantageously from 200 to 700 ml/100 g, particularly advantageously from 300 to 520 ml/100 g, in particular in the range from 300 to 345 ml/100 g or 470 to 520 ml/100 g. The particle sizes of the carbon blacks in the polymer matrix of the molding composition are in the range from 0.01 to 2 μm, advantageously in the range from 0.05 to 0.15 μm. The primary particle size is in the range from 0.02 to 0.05 μm. The carbon blacks used have agglomerate particle sizes of from 10-50 μm, densities of from 0.1 to 1.6 g/ml, electrical resistivities in the range from 10 to 80*10⁻⁴ Ωcm, advantageously from 30 to 50*10⁻⁴ Ωcm, in particular 40*10⁻⁴ Ωcm, and low thermal conductivities of less than 0.15 W/mK, in particular 0.07 W/mK. The electrical conductivities as a function of filler content (the shape of the percolation curve) can be modified via the structure of the carbon black in the matrix, and the skilled worker can therefore easily control and optimize the product properties by using changes in the shear energy introduced and in the residence time.

The graphite used according to the invention is a graphite with no strongly developed structure. The specific surface area of the graphite is from 1 to 35 m²/g, advantageously from 2 to 20 m²/g, particularly advantageously from 3 to 10 m²/g. The particle size of the graphite used is from 1 to 1100 μm, with a median particle size of from 50 to 450 μm. The particle size is advantageously in the range from 10 to 1000 μm, particularly advantageously from 10 to 800 μm, very particularly advantageously from 10 to 500 μm. The median particle size is advantageously in the range from 100 to 300 μm, and is particularly advantageously 200 μm. The numbers given here are uncorrected values, and have to be corrected upward or downward using the tolerances of the test methods used. The graphite used moreover has high thermal conductivities above 100 W/mK, advantageously above 180 W/mK, particularly advantageously above 200 W/mK. The electrical resistivities are generally from 5 to 15*10⁻⁴ Ωcm, advantageously below 10*10⁻⁴ Ωcm, in particular about 8*10⁻⁴ Ωcm.

The metal powder used may in principle be any of the metal powders with a defined particle size and particle distribution.

The metal powder used advantageously has an apparent density to ISO 3923/1 of from 1 to 4 g/ml, advantageously from 2.7 to 3.2 g/ml, particularly advantageously from 2.8 to 3.1 g/ml.

The metal powder used has a fraction of 5% by weight, advantageously from 4 to 1% by weight, particularly advantageously below 1% by weight, in particular 0.8% by weight, with particle sizes up to 45 μm. The proportion of particle sizes greater than 45 μm is more than 95% by weight, advantageously from 96 to 99% by weight, particularly advantageously above 99% by weight, in particular 99.2% by weight. Examples of metal powders which may advantageously be used are aluminum, chromium, iron, gold, iridium, cobalt, copper, magnesium, manganese, molybdenum, nickel, niobium, osmium, palladium, platinum, rhenium, rhodium, samarium, silver, titanium, vanadium, bismuth, tungsten, zinc, tin, alloys or mixtures made from two or more of these metals, and include mixtures and/or alloys which are liquid under the conditions of processing. Alloys which may be mentioned here, merely by way of example, are brass, steel, V2A steel and V4A steel.

According to the invention it is possible to use the polyarylene sulfides known per se. Suitable materials are described, by way of example, in Saechtling, Kunststoff-Taschenbuch [Plastics handbook], Hanser-Verlag, 27th-edition, on pages 495-498, and this citation is incorporated herein by way of reference. It is advantageous to use thermoplastic polyarylene sulfides. Polyphenylene sulfide, PPS, is particularly advantageous.

Polyarylene sulfides may be prepared using dihalogenated aromatic compounds. Preferred dihalogenated aromatic compounds are p-dichlorobenzene, m-dichlorobenzene, 2,5-dichlorotoluene, p-dibromobenzene, 1,4-dichloronaphthalene, 1-methoxy-2,5-dichlorobenzene, 4,4′-dichlorobiphenyl, 3,5-dichlorobenzoic acid, 4,4′-dichlorodiphenyl ether, 4,4′-dichlorodiphenyl sulfone, 4,4′-dichlorodiphenyl sulfoxide, and 4,4′-dichlorodiphenyl ketone. It is also possible to use small amounts of other halogenated compounds, such as trihalogenated aromatics, in order to exert a specific effect on the properties of the polymer.

According to the invention, the polyarylene sulfide used preferably comprises polyphenylene sulfide. Polyphenylene sulfide (PPS) is a semicrystalline polymer with the general formula:

where n>1, and where the molar mass (M_(w)) of the polymer is above 200 g/mol.

According to the invention, use may also be made of the liquid-crystalline plastics (LCPs) known per se. There are no restrictions on the type of materials used, but advantageous materials are those which are capable of thermoplastic processing. Particularly suitable materials are described, by way of example, in Saechtling, Kunststoff-Taschenbuch, Hanser-Verlag, 27th edition, on pages 517-521, and this citation is incorporated herein by way of reference. Materials which may be used advantageously are polyterephthalates, polyisophthalates, PET-LCP, PBT-LCP, poly(m-phenyleneisophthalamide), PMPI-LCP, poly(p-phenylenephthalimide), PPTA-LCP, polyarylates, PAR-LCP, polyester carbonates, PEC-LCP, polyazomethines, polythioesters, polyesteramides, polyesterimides. Particularly advantageous materials are p-hydroxybenzoic-acid-based liquid-crystalline plastics, such as copolyesters or copolyesteramides. Liquid-crystalline plastics whose use is very particularly advantageous are generally polyesters which are fully aromatic and form anisotropic melts and which have average molar masses (M_(w)=weight-average) of from 2000 to 200000 g/mol, preferably from 3500 to 50000 g/mol, and in particular from 4000 to 30000 g/mol. A suitable group of liquid-crystalline polymers is described in U.S. Pat. No. 4,161,470, which is incorporated herein by way of reference. These are naphthoyl copolyesters with repeat structural units of the formulae I and II

where the T selected is an alkyl radical, an alkoxy radical, in each case having from 1 to 4 carbon atoms, or a halogen, preferably chlorine, bromine or fluorine, and s is zero or an integer 1, 2, 3 or 4, and if there is more than one radical T these are independent of one another and identical or different. The naphthoyl copolyesters contain from 10 to 90 mol %, preferably from 25 to 45 mol %, of structural units of the formula I, and from 90 to 10 mol %, preferably from 85 to 55 mol %, of structural units of the formula II, where the proportions of structural units of the formulae I and II together give 100 mol %.

EP-A-0 278 066—and U.S. Pat. No. 3,637,595, which are incorporated herein by way of reference, describe other liquid-crystalline polyesters suitable for the molding compositions of the invention, and mention oxybenzoylcopolyesters containing structural units of the formulae III, IV and V, where one or more of the structural units specified may be present in each case.

In the formula III, IV and V, k is zero or 1, v, w, and x are integers equal to or greater than 1, the D selected is an alkyl radical having from 1 to 4 carbon atoms, an aryl radical, an aralkyl radical having from 6 to 10 carbon atoms in each case, or a halogen, such as fluorine, chlorine or bromine, and s is as defined above, and if there is more than one radical D, these are independent of one another and identical or different. The total of the indices v, w and x is from 30 to 600. The oxybenzoylcopolyesters generally contain from 0.6 to 60 mol %, preferably from 8 to 48 mol %, of structural units of the formula II, from 0.4 to 98.5 mol %, preferably from 5 to 85 mol %, of structural units of the formula IV, and from 1 to 60 mol %, preferably from 8 to 48 mol %, of structural units of the formula V, where the proportions of the structural units of the formulae III, IV and V together give 100 mol %.

Other suitable copolyesters are those which contain only structural units of the formulae III and V. These liquid-crystalline polymers generally contain from 40 to 60 mol % of the structural units of the formula III, and from 60 to 40 mol % of structural units of the formula V. Preference is given here to a molar ratio of 1:1. Polyesters of this type are described, by way of example, in U.S. Pat. No. 4,600,765; U.S. Pat. No. 4,614,790 and U.S. Pat. No. 4,614,791, which are incorporated herein by way of reference.

Other suitable copolyesters are those which, besides the structural units selected from the formulae III to V, also contain those of the formulae I and/or II, e.g. with from 15 to 1 mol % of structural units of the formula I, from 50 to 79 mol % of those of formula II, from 20 to 10 mol % of those of formula III, and from 20 to 10 mol % of those of formula V.

Other liquid-crystalline plastics which can be used advantageously for the molding compositions of the invention are copolyesteramides which, besides one or more structural units of the formulae I to V, also have at least one structural unit of the formula VI or VII

where R may be phenylene or naphthylene, Z may be a CO or O (oxygen) group, and T and s are as defined above. The liquid-crystalline plastics which are suitable may be used individually or as mixtures.

Other suitable liquid-crystalline plastics also contain, besides the structural units I to VII, at least one structural unit VIII

where T and s are as defined above.

Either the liquid-crystalline plastic or the polyarylene sulfide may comprise conventional additives and reinforcing materials, for example fibers, in particular glass fibers, carbon fibers, aramid fibers, mineral fibers, processing aids, polymeric lubricants, lubricants with external and/or internal lubricant action, ultrahigh-molecular-weight polyethylene (UHMWPE), polytetrafluoroethylene (PTFE) or a graft copolymer which is a product made from an olefin polymer and from an acrylonitrile-styrene copolymer in a graft reaction, antioxidants, adhesion promoters, waxes, nucleating agents, mold-release agents, glass beads, mineral fillers, such as chalk, calcium carbonate, wollastonite, silicon dioxide, talc, mica, montmorillonite, organically modified or unmodified, organically modified or unmodified phyllosilicates, materials forming nanocomposites with the liquid-crystalline plastic or with the polyarylene sulfide, or nylon nanocomposites, or mixtures of the abovementioned substances.

The lubricant used may be a mixture made from a lubricant with external lubricant action and from a lubricant with internal lubricant action. The mixing ratio of lubricant with internal lubricant action to lubricant with external lubricant action may be from 0:100 to 100:0 parts by weight. The lubricant used with predominantly external lubricant action may be solid and/or liquid paraffins, montanic esters, partially hydrolyzed montanic esters, stearic acids, polar and/or non-polar polyethylene waxes, poly-a-olefin oligomers, silicone oils, polyalkylene glycols or perfluoroalkyl ethers. Soaps and esters, including those partially hydrolyzed, are also lubricants with both external and internal lubricant action. Preference is given to the use of a high-molecular weight polyethylene wax which has been oxidized and is therefore polar. This improves tribological properties and permits a less pronounced fall-off in mechanical properties. Stearyl stearate is preferably used as lubricant with predominantly internal lubricant action. Paraffins, solid or liquid, stearic acids, polyethylene waxes, non-polar or polar, poly-α-olefin oligomers, silicone oils, polyakylene glycols and perfluoroalkyl ethers are lubricants with external lubricant action. Soaps and esters, including those partially hydrolyzed, are lubricants with both external and internal lubricant action. Montanic esters and partially hydrolyzed montanic esters are lubricants with external lubricant action.

The preferred oxidized polyethylene wax is a high-molecular-weight polar wax, and generally has an acid value of from 12 to 20 mg KOH/g, and a viscosity of from 3000 to 5000 mPa·s at 140° C.

Lubricants which should be mentioned with predominantly internal lubricant action are: fatty alcohols, dicarboxylic esters, fatty esters, fatty acids, fatty acid soaps, fatty amides, wax esters, and stearyl stearates, the last-named being preferred. Lubricants are described in Gächter and Müller, “Taschenbuch der Kunststoff-Additive” [Plastics additives handbook], 3rd edition, Carl Hanser Verlag, Munich/Vienna, 1994, pages 478-504, and this citation is incorporated herein by way of reference.

The molding compositions of the invention may be prepared and processed by the conventional processes for thermoplastics, such as kneading, extrusion, injection molding, transfer molding and compression molding.

The median particle dimensions of the graphite are decisive for developing good electrical conductivity in the component. In order to avoid excessive reduction of these particle dimensions by high shear forces, it is necessary to use particularly non-aggressive preparation and shaping processes. As stated above, use is made of graphite types with a median particle size in the range from 50 to 450 μm, advantageously in the range from 100 to 300 μm, particularly advantageously of 200 μm, incorporated into the matrix polymers of the invention using a filler content of below 85% by weight, advantageously of below or equal to 80% by weight, particularly advantageously from 60 to 80% by weight.

In relation to the properties of the final product, it has proven particularly advantageous to use processes in which the preparation and shaping step have been combined to give a single-stage process. Examples of this are injection molding-compounding with or without an injection-compression molding unit and the melt-application compression-molding process, which is based on the combination of a preparation assembly (single-screw, twin-screw or the like) and a compression-molding unit. All of these single-stage processes are processes well-known in the art or are otherwise known from the literature.

Injection molding-compounding without injection-compression molding unit: see R. Jensen: Synergien intelligent nutzen—IMC-Spritzgiesscompounder erhoht Wertschöpfung [Intelligent utilization of synergies—IMC Injection-molding compounder increases value-added]; Kunststoffe plast europe, 9/2001; and also R. Jensen: Synergie schafft neue Technologie [Synergy creates new technology]; Kunststoffe plast europe 10/2001, incorporated herein by way of reference.

Injection-molding compounding with injection-compression molding unit (known as injection-compression molding): see F. Johannaber, W. Michaeli: Handbuch Spritzgiessen [Injection molding handbook], Carl Hanser-Verlag, Munich (2001), ISBN 3-446-15632-1, p. 417; and also H. Saechtling:

Kunststofftaschenbuch [Plastics handbook], 27th edition, Carl Hanser-Verlag, Munich (1998), ISBN 3446-19054-6, p. 226, incorporated herein by way of reference.

Melt-application compression molding: T. Hofer: Fillflow—A comparison between simulation and experiment in the case of the extrusion compression moulding. Proceedings of the 3rd ESAFORM Conference on Material Forming, Stuttgart (2000); ISBN 3-00-005861-3; and also R. D. Krause, Dissertation, Stuttgart University, Process Technology Faculty, Institut Kunststofftechnologie [Institute for Plastics Technology] (1998); Modellierung und Simulation rheologisch-thermodynamischer Vorgange bei der Herstellung grossflatchiger thermoplastischer Formteile mittels Kompressionsformverfahren [Modeling and simulation of rheological-thermodynamic processes during the production of large-surface area thermoplastic moldings by compression-molding processes], incorporated herein by way of reference.

In particular, the use of an injection-molding compounder (IMC) proves advantageous, because preparation and shaping of the filled system take place in one step, without reheating. If, in addition, an injection-compression molding unit or compression molding unit is utilized, the damage to particles is dramatically reduced when comparison is made with simple injection molding. This damage to particles, brought about by attack by high shear stresses and deformation rates during injection into the mold cavity at high injection rates reduces component conductivity significantly by factors of from 3 to 10. Use of the injection-compression molding unit permits non-aggressive injection of a shot of melt within the cavity, and permits the final shaping of the component to be brought about by fully closing the cavity.

However, another process which has proven particularly advantageous is shaping via compression, using a compression mold (positive mold), the process known as compression molding, this likewise being a process well-known in the prior art, and widely known from the literature.

Compression molding: Kunststofflaschenbuch [Plastics handbook], 25th edition, Carl Hanser-Verlag, Munich (1998), ISBN 3-446-16498-7, pp. 113 et seq., incorporated herein by way of reference.

It has proven particularly advantageous here for the molding compositions of the invention to be pre-ground, e.g. on a grinder, or jaw crusher, or in a ball mill or pinned disc mill, after the preparation step and before the compression molding process.

Particle sizes of from 1500 to 50 μm, preferably from 1000 to 100 μm, and particularly preferably from 800 to 150 μm, for the pre-ground molding compositions are particularly advantageous for shaping via compression molding.

The molding compositions of the invention may be used in any of the sectors where there is a need for conductive plastics. The molding compositions of the invention may be used advantageously for parts of fuel cells, in particular parts of end plates of a fuel cell, or for end plates or bipolar plates of fuel cells. The bipolar plates, end plates or parts of end plates produced from the molding compositions of the invention are suitable for producing high-output fuel cells with a specific output greater than one kilowatt per kilogram, and can achieve specific electrical conductivities above 100 S/cm, and are chemically resistant to all of the materials used in operating a fuel cell, for example mains water, demineralized water, acids, hydrogen, methanol, and are also impermeable to these. In this connection see also the German Patent Application with file reference No. 10064656.5-45. The heat distortion temperature of the molding compositions of the invention is above 130° C. at 1.82 MPa test load. Their flexural strength is from 30 to 50 MPa, and is therefore markedly above the minimum requirement of 20 MPa. Since it is possible to use conventional injection molding or injection-compression molding, and no machining is needed, high production rates can be achieved. By selecting suitable filler systems, it is possible to obtain components which have the same electrical properties but have from 10 to 23% reduction in the proportion of filler, and from 3 to 10% reduction in component density, while mechanical and rheological properties are improved. Components manufactured from the molding compositions of the invention are therefore particularly suitable for application in mobile fuel cells as well as for use in stationary fuel cells.

EXAMPLES

In the examples, molding compositions were prepared from liquid crystalline plastic (Vectra A 950, Ticona GmbH, Frankfurt). The carbon black used was Ketjenblack EC-600JD from Akzo Nobel, with a dibutyl phthalate value of from 480 to 150 ml/100 g, iodine absorption of from 1000 to 1150 mg/g, and apparent density of from 100 to 120 kg/m³. This carbon black comprises 7% of particles of size below 125 μm. The graphite used was Thermocarb CF-300 from Conoco. The zinc powder used was a zinc powder from Eckart Dorn, which has a median particle size of 20 μm, apparent density of 3.06 g/ml, and 0.8% of particles of size below 45 μm (measured to ISO 4497). In the comparative examples, the filler loading is identical with the filling content. The carbon black content in carbon black/graphite mixtures or carbon black/zinc mixtures from Tables 1-5 was 7.5% by weight, and the filler loading was achieved by increasing the proportion of graphite or of zinc.

Table 1 lists the results of molding compositions prepared using a Buss Ko-Kneader (L/D=15), and the examples in Table 2 used a Werner & Pfleiderer ZSK 25 with an UD ratio of 42. The resistance measurements were carried out to ISO 3915-1981 on round extrudates.

The results from Table 1 are shown graphically in FIG. 1, and the results from Table 2 in FIG. 2, plotted on a semilogarithmic scale.

TABLE 1 Filler loading Resistivity Density/ φM/% RD/Ω cm g/ml Carbon black Comparative Example 1 4.76 68.45 1.33 Comparative Example 2 6.98 20.67 1.3 Comparative Example 3 9.09 4.87 1.27 Comparative Example 4 11.11 2.59 1.24 Comparative Example 5 13.04 1.12 1.21 Graphite Comparative Example 6 33.33 999.72 1.68 Comparative Example 7 42.86 29.33 1.76 Comparative Example 8 50 11.82 1.82 Comparative Example 9 55.56 4.16 1.87 Comparative Example 10 60 2.18 1.9 Comparative Example 11 66.67 0.67 1.96 Comparative Example 12 75 0.36 2.03 Carbon black/graphite Example 13 14.89 11.77 1.39 Example 14 18.37 7.59 1.42 Example 15 21.57 6.69 1.46 Example 16 24.53 4.34 1.49 Example 17 27.28 3.49 1.51 Example 18 61.47 0.35 1.86 Example 19 67.48 0.25 1.93 Example 20 75.46 0.19 1.99

TABLE 2 Filler loading Resistivity φM/% RD/Ω cm Density/g/ml Carbon black Comparative Example 21 4.76 137.12 1.34 Comparative Example 22 6.98 22.94 1.3 Comparative Example 23 9.09 4.01 1.28 Comparative Example 24 11.11 2.86 1.24 Comparative Example 25 13.04 1.53 1.21 Graphite Comparative Example 26 33.33 1021 1.68 Comparative Example 27 50 13.42 1.83 Comparative Example 28 60 2.49 1.9 Comparative Example 29 67 1.24 1.97 Comparative Example 30 75 0.6 2.04 Zinc Comparative Example 31 78.57 575.12 2.64 Comparative Example 32 80 266.45 2.67 Comparative Example 33 81.25 0.72 2.69 Comparative Example 34 82.35 0.01 2.71 Comparative Example 35 83.33 0.005 2.72 Carbon black/graphite Example 36 14.89 14.68 1.39 Example 37 21.57 7.5 1.46 Example 38 27.28 3.55 1.51 Example 39 40.29 2.12 1.64 Example 40 51.81 1.29 1.76 Example 41 71.83 0.136 1.98 Example 42 75.31 0.086 2.01 Example 43 76.74 0.077 2.03 Carbon black/zinc Example 44 51.22 361.41 2.17 Example 45 60.78 85.77 2.33 Example 46 75.3 0.04 2.58 Example 47 80.19 0.01 2.66 Example 48 81.13 0.002 2.68

The volume resistivities given in the examples and comparative examples above were determined on the round extrudates which emerged from the die of the compounding assembly. Depending on the pressure level during the production of bipolar plates, there is a reduction in these values by a factor of from about 5 to 20 in the case of the carbon black/graphite molding compositions. This is attributable to effects of compacting the material and by their properties (non-Newtonian flow behavior with yield point), since cracks can be observed to form on emergence from the die, and this increases resistivity. This is clear from the resistivities of bipolar plates produced from molding compositions of Examples 41 and 43. These are listed as Examples 49 and 50 in Table 3.

TABLE 3 Mass Mass flow Mass flow Resistivity Resistivity flow of of carbon of of of polymer/ black/ graphite/ compound/ plate/ Example kg/h kg/h kg/h Ω cm Ω cm 49 1.425 0.075 3.75 0.136 0.0187 50 1.425 0.075 4.5 0.077 0.0042

Further molding compositions with varied carbon black contents were prepared systematically, and used for measurements on round extrudates and measurements on a bipolar plate preform.

A P300P laboratory press with positive mold from the company Collin was used to produce molded bipolar plates by compression molding. The area of the plates was 160*160 mm. The mixtures of the raw materials were heated to 300° C. in the mold, then compressed at from 100 to 250 bar for 5 min and then cooled from 300° C. to 40° C., i.e. at ˜0.3° C./s, at from 50 to 125 bar during a period of 900 s.

The results of measurements on liquid-crystalline plastic are listed in Table 4 and plotted on a semilogarithmic scale in FIG. 3. The results of measurements on a polyphenylene sulfide (Fortron, Ticona GmbH, Frankfurt) are listed in Table 5 and shown graphically in FIG. 4. In the comparative examples, filler loading is the same as filler content. In the case of carbon black/graphite mixtures the figure given is the carbon black content, and the filler loading was achieved by adding graphite.

TABLE 4 RD RD (round (bipolar φM/% extrudate)/ plate)/ Density/ by weight Ω cm Ω cm g/ml LCP, carbon black only Comparative Example 51 5 35.49 13.45 1.42 Comparative Example 52 7.5 5.24 1.71 1.43 Comparative Example 53 10 2.04 0.51 1.44 Comparative Example 54 12.5 1.84 0.21 1.45 Comparative Example 55 15 1.00 0.20 1.46 LCP, graphite only Comparative Example 56 50 5.24 0.18 1.72 Comparative Example 57 60 0.63 0.05 1.81 Comparative Example 58 66.6 0.52 0.027 1.87 Comparative Example 59 71.4 0.33 0.019 1.91 Comparative Example 60 75 0.25 0.009 1.95 LCP, 5% of carbon black and graphite Example 61 51.17 1.26 0.064 1.71 Example 62 61.24 0.40 0.019 1.80 Example 63 67.7 0.21 0.012 1.86 Example 64 72.45 0.13 0.009 1.92 Example 65 75.87 0.12 0.005 1.95 LCP, 7.5% of carbon black and Graphite Example 66 51.9 0.51 0.032 1.71 Example 67 61.89 0.17 0.016 1.81 Example 68 68.365 0.10 0.007 1.87 Example 69 72.99 0.10 0.006 1.92 LCP, 10% of carbon black and Graphite Example 70 52.66 0.18 0.022 1.73 Example 71 62.66 0.12 0.012 1.82 LCP, 12.5% of carbon black Example 72 53.275 0.14 0.019 1.74

TABLE 5 RD (round φM/% extrudate)/ RD (bipolar Density/ by weight Ω cm plate)/Ω cm g/ml PPS, carbon black only Comparative Example 73 5 111.77 13.45 1.42 Comparative Example 74 10 1.00 1.71 1.43 Comparative Example 75 15 0.89 0.51 1.44 PPS, graphite only Comparative Example 76 50 5.26 0.99 1.72 Comparative Example 77 60 0.93 0.26 1.81 Comparative Example 78 66.6 0.64 0.093 1.87 Comparative Example 79 71.4 0.36 0.085 1.91 Comparative Example 80 75 0.24 0.076 1.95 PPS, 5% of carbon black and graphite Example 81 51.17 0.32 0.048 1.69 Example 82 61.24 0.17 0.035 1.77 Example 83 67.7 0.12 0.023 1.84 Example 84 72.45 0.09 0.013 1.90 Example 85 75.87 0.10 0.009 1.94 PPS, 10% of carbon black and graphite Example 86 52.66 0.08 0.027 1.70 Example 87 62.56 0.06 0.01 1.78 Example 88 68.86 0.04 0.008 1.85 PPS, 15% of carbon black and graphite Example 89 53.93 0.03 0.014 1.70

The effect of the median graphite particle size on the resultant electrical resistance values is shown by FIGS. 5, 6, and 7. The values measured on bipolar plates are given in Table 6 and FIG. 5. The process parameters were exactly identical, and the only variation consisted in the particle size of the graphite filler (FIGS. 6 and 7). Use was made of what is known as a standard graphite Thermocarb CF-300 with a median particle size (FIG. 6) of ˜130 μm and of what is known as a micronized graphite Thermocarb CF-300 with a median particle size (FIG. 7) of ˜10 μm. It is very apparent that the volume resistivities achievable using standard graphite are markedly lower. In addition, the conductivity gain when utilizing binary carbon black/graphite filler systems here becomes clear.

A P300P laboratory press with positive mold from the company Collin was used to produce molded bipolar plates by compression moldings. The area of the plates was 160*160 mm. The mixtures of the raw materials were heated to 300° C. in the mold, then compressed at from 100 to 250 bar for 5 min and then cooled from 300° C. to 40° C., i.e. at ˜0.3° C./s, at from 50 to 125 bar during a period of 900 s.

The column “Constitution” in Tables 6 to 10 is to be interpreted as follows:

The total of the proportions by weight of plastic (LCP or PPS) and carbon black (CB) is always 100%. LCPIR-5/G-195 therefore denotes a mixture of 95% by weight of LCP, 5% by weight of carbon black, and 195% by weight of graphite (G). The filler loading by weight φM is calculated as follows: φM=(weight of carbon black+weight of graphite)/(weight of plastic+weight of carbon black+weight of graphite).

TABLE 6 φM/% Resistivity Constitution by weight RD/Ω cm LCP, Standard Graphite CF-300 Comparative Example 90 LCP/R-0/G-300 75 0.0043 Comparative Example 91 LCP/R-0/G-250 71.4 0.0048 Comparative Example 92 LCP/R-0/G-200 66.6 0.0065 Comparative Example 93 LCP/R-0/G-150 60 0.0105 Comparative Example 94 LCP/R-0/G-100 50 0.0277 Example 95 LCP/R-5/G-295 75.87 0.0028 Example 96 LCP/R-5/G-245 72.45 0.0033 Example 97 LCP/R-5/G-195 67.7 0.0045 Example 98 LCP/R-5/G-145 61.24 0.0063 Example 99 LCP/R-5/G-95 51.17 0.0127 Example 100 LCP/R-7.5/G-267.5 74.48 0.0034 Example 101 LCP/R-7.5/G-242.5 72.99 0.0035 Example 102 LCP/R-7.5/G-192.5 68.37 0.0041 Example 103 LCP/R-7.5/G-142.5 61.89 0.0060 Example 104 LCP/R-7.5/G-92.5 51.9 0.0095 Example 105 LCP/R-10/G-190 68.96 — Example 106 LCP/R-10/G-140 62.56 0.0056 Example 107 LCP/R-10/G-90 52.66 0.0075 Example 108 LCP/R-12.5/G-87.5 53.28 0.0073 LCP, Micronized graphite CF-300 Comparative Example 109 LCP/R-0/G-400 80 0.0050 Comparative Example 110 LCP/R-0/G-375 78.95 0.0055 Comparative Example 111 LCP/R-0/G-350 77.78 0.0068 Comparative Example 112 LCP/R-0/G-325 76.47 0.0075 Comparative Example 113 LCP/R-0/G-300 75 0.0086 Comparative Example 114 LCP/R-0/G)-250 71.4 0.0110 Comparative Example 115 LCP/R-0/G-200 66.6 0.0160 Comparative Example 116 LCP/R-0/G-150 60 0.0308 Comparative Example 117 LCP/R-0/G-100 50 0.0865 Example 118 LCP/R-5/G-295 75.87 0.0066 Example 119 LCP/R-5/G-245 72.45 0.0083 Example 120 LCP/R-5/G-195 67.7 0.0118 Example 121 LCP/R-5/G-145 61.24 0.0169 Example 122 LCP/R-5/G-95 51.17 0.0392 Example 123 LCP/R-7.5/G-267.5 74.48 0.0053 Example 124 LCP/R-7.5/G-242.5 72.99 0.0073 Example 125 LCP/R-7.5/G-192.5 68.37 0.0103 Example 126 LCP/R-7.5/G-142.5 61.89 0.0139 Example 127 LCP/R-7.5/G-92.5 51.9 0.0295 Example 128 LCP/R-10/G-190 68.96 0.0071 Example 129 LCP/R-10/G-140 62.56 0.0105 Example 130 LCP/R-10/G-90 52.66 0.0194 Example 131 LCP/R-12.5/G-87.5 53.28 —

The effect of preparation parameters and specimen homogeneity on the volume resistivities are given for LCP bipolar plates in FIG. 8 and Table 7, and those for PPS bipolar plates are given in FIG. 9 and Table 8.

A P300P laboratory press with positive mold from the company Collin was used to produce molded bipolar plates by compression molding. The area of the plates was 160*160 mm. The mixtures of the raw materials were heated to 300° C. in the mold, then compressed at from 100 to 250 bar for 5 min and then cooled to 40° C. at 0.3° C./s, at from 50 to 125 bar.

The substantial process-technology difference between “standard preparation” and “optimized preparation” can be found in the design of the screws. The latter screw configuration comprises, after the graphite filler feed(s), conveying elements with 2× change in axial compression, and includes few or no functional elements (mixing elements and kneading elements).

The granular material for the compression-molding of the molded plates in the “optimized preparation” version was moreover ground, using a jaw crusher, prior to the shaping step, and fractionated, using a 1000 μm sieve, in order to ensure that the bipolar plates had better homogeneity. This improves not only mechanical but also electrical properties, because firstly no grain boundaries remain in the test specimen and secondly polymer-coated fillers (in particular graphite) are fragmented.

It is apparent not only for LCP bipolar plates but also for PPS bipolar plates, that firstly non-aggressive preparation ensures that volume resistivities are lower and secondly that the material forming an initial charge to generate the component in the cavity has to have maximum homogeneity, i.e. should preferably not be in the form of granular material but in the form of a unitary preform when introduced into the shaping unit “in a single shot”, preferably “without reheating”.

TABLE 7 φM/% by Resistivity Constitution weight RD/Ω cm LCP, Standard Graphite CF-300 Optimized preparation Comparative Example 132 LCP/R-0/G-300 75 0.0043 Comparative Example 133 LCP/R-0/G-250 71.4 0.0048 Comparative Example 134 LCP/R-0/G-200 66.6 0.0065 Comparative Example 135 LCP/R-0/G-150 60 0.0105 Comparative Example 136 LCP/R-0/G-100 50 0.0277 Example 137 LCP/R-5/G-295 75.87 0.0028 Example 138 LCP/R-5/G-245 72.45 0.0033 Example 139 LCP/R-5/G-195 67.7 0.0045 Example 140 LCP/R-5/G-145 61.24 0.0062 Example 141 LCP/R-5/G-95 51.17 0.0127 Example 142 LCP/R-7.5/G-267.5 74.48 0.0034 Example 143 LCP/R-7.5/G-242.5 72.99 0.0035 Example 144 LCP/R-7.5/G-192.5 68.37 0.0041 Example 145 LCP/R-7.5/G-142.5 61.89 0.0060 Example 146 LCP/R-7.5/G-92.5 51.9 0.0095 Example 147 LCP/R-10/G-190 68.96 — Example 148 LCP/R-10/G-140 62.56 0.0056 Example 149 LCP/R-10/G-90 52.66 0.0075 Example 150 LCP/R-12.5/G-87.5 53.28 0.0073 LCP, Standard graphite CF-300 Standard preparation Comparative Example 151 LCP/R-0/G-300 75 0.0093 Comparative Example 152 LCP/R-0/G-250 71.4 0.0190 Comparative Example 153 LCP/R-0/G-200 66.6 0.0275 Comparative Example 154 LCP/R-0/G-150 60 0.0497 Comparative Example 155 LCP/R-0/G-100 50 0.1819 Example 156 LCP/R-5/G-295 75.87 0.0049 Example 157 LCP/R-5/G-245 72.45 0.0087 Example 158 LCP/R-5/G-195 67.7 0.0122 Example 159 LCP/R-5/G-145 61.24 0.0195 Example 160 LCP/R-5/G-95 51.17 0.0642 Example 161 LCP/R-7.5/G-267.5 74.48 — Example 162 LCP/R-7.5/G-242.5 72.99 0.0062 Example 163 LCP/R-7.5/G-192.5 68.37 0.0066 Example 164 LCP/R-7.5/G-142.5 61.89 0.0155 Example 165 LCP/R-7.5/G-92.5 51.9 0.0319 Example 166 LCP/R-10/G-190 68.96 — Example 167 LCP/R-10/G-140 62.56 0.0124 Example 168 LCP/R-10/G-90 52.66 0.0221 Example 169 LCP/R-12.5/G-87.5 53.28 0.0194

TABLE 8 φM/% by Resistivity Constitution weight RD/Ω cm PPS, Standard Graphite CF-300 Optimized preparation Example 170 PPS/R-15/G-85 53.93 0.0067 Example 171 PPS/R-10/G-190 68.86 0.0055 Example 172 PPS/R-10/G-140 62.56 0.0053 Example 173 PPS/R-10/G-90 52.66 0.0112 Example 174 PPS/R-5/G-295 75.87 0.0065 Example 175 PPS/R-5/G-245 72.45 0.0074 Example 176 PPS/R-5/G-195 67.7 0.0088 Example 177 PPS/R-5/G-145 61.24 0.0106 Example 178 PPS/R-5/G-95 51.17 0.0196 Comparative Example 179 PPS/R-0/G-300 75 0.0198 Comparative Example 180 PPS/R-0/G-250 71.4 0.0193 Comparative Example 181 PPS/R-0/G-200 66.6 0.0332 Comparative Example 182 PPS/R-0/G-150 60 0.0372 Comparative Example 183 PPS/R-0/G-100 50 0.0685 PPS, Standard Graphite CF-300 Standard preparation Example 184 PPS/R-15/G-85 53.93 0.0142 Example 185 PPS/R-10/G-190 68.86 0.0080 Example 186 PPS/R-10/G-140 62.56 0.0096 Example 187 PPS/R-10/G-90 52.66 0.0271 Example 188 PPS/R-5/G-295 75.87 0.0088 Example 189 PPS/R-5/G-245 72.45 0.0132 Example 190 PPS/R-5/G-195 67.7 0.0229 Example 191 PPS/R-5/G-145 61.24 0.0346 Example 192 PPS/R-5/G-95 51.17 0.0481 Comparative Example 193 PPS/R-0/G-300 75 0.0762 Comparative Example 194 PPS/R-0/G-250 71.4 0.0849 Comparative Example 195 PPS/R-0/G-200 66.6 0.0932 Comparative Example 196 PPS/R-0/G-150 60 0.2602 Comparative Example 197 PPS/R-0/G-100 50 0.9852

The effect of the processing on the volume resistivity is shown in FIG. 10 for LCP formulations and in FIG. 11 for PPS formulations. The values measured are given in Tables 9 and 10.

To produce test plates by compression molding, the process parameters used were the same as those for production of molded bipolar plates (Table 7 & 8). For the injection molding process, use was made of machines from the company Arburg (Allrounder) and Krauss-Maffei. The processing recommendations for Vectra (LCP) and Fortron (PPS) in the product brochures from Ticona GmbH were heeded. To produce test plates via melt-application compression molding, a P300P laboratory press with positive mold from the company Collin was supplied directly with the melt extrudate emerging from an extruder (ZSK 25 from the company Werner & Pfleiderer). The area of the plate was 160*160 mm. The melt extrudate temperature was from 300 to 320° C., and the temperature of the cavity of the press was 300° C. The melt was compressed at from 100 to 250 bar for 5 min and then cooled to 40° C. at 0.3° C./s at from 50 to 125 bar.

It is apparent for both materials, LCP and PPS, that test plates produced by means of standard injection molding have substantially higher volume resistivities than compression-molded test specimens. The values measured for the specimens produced by melt-application compression molding, in this case of flat-extrudate sections, into a compression molding cavity are somewhat poorer than those for compression molding (FIG. 10). From FIGS. 12 and 13 it is seen that the resumption of shear-energy-induced melting in the plastifying assembly of the injection molding machine—and in particular the shear processes and deformation processes during injection of the plastifed mass into the cavity, causes damage to the graphite particles, whereas this is not to be found during compression molding or melt-application compression molding or injection-compression molding, and in particular not during processes of this type which proceed in a single stage.

TABLE 9 φM/% by Resistivity Constitution weight RD/Ω cm LCP, Standard Graphite CF-300 Compression molding Comparative Example 198 LCP/R-0/G-300 75 0.0043 Comparative Example 199 LCP/R-0/G-250 71.4 0.0048 Comparative Example 200 LCP/R-0/G-200 66.6 0.0065 Comparative Example 201 LCP/R-0/G-150 60 0.0105 Comparative Example 202 LCP/R-0/G-100 50 0.0277 Example 203 LCP/R-5/G-295 75.87 0.0028 Example 204 LCP/R-5/G-245 72.45 0.0033 Example 205 LCP/R-5/G-195 67.7 0.0045 Example 206 LCP/R-5/G-145 61.24 0.0063 Example 207 LCP/R-5/G-95 51.17 0.0127 Example 208 LCP/R-7.5/G-267.5 74.48 0.0034 Example 209 LCP/R-7.5/G-242.5 72.99 0.0035 Example 210 LCP/R-7.5/G-192.5 68.37 0.0041 Example 211 LCP/R-7.5/G-142.5 61.89 0.0060 Example 212 LCP/R-7.5/G-92.5 51.9 0.0095 Example 213 LCP/R-10/G-140 62.56 0.0056 Example 214 LCP/R-10/G-90 52.66 0.0075 Example 215 LCP/R-12.5/G-87.5 53.28 0.0073 Example 216 LCP/R-5/G-0 5 13.45 Example 217 LCP/R-7.5/G-0 7.5 1.71 Example 218 LCP/R-10/G-0 10 0.51 Example 219 LCP/R-12.5/G-0 12.5 0.21 LCP, Standard graphite CF-300 Injection molding Comparative Example 220 LCP/R-0/G-300 75 0.0177 Comparative Example 221 LCP/R-0/G-250 71.4 0.0252 Comparative Example 222 LCP/R-0/G-200 66.6 0.0427 Comparative Example 223 LCP/R-0/G-150 60 0.0970 Comparative Example 224 LCP/R-0/G-100 50 0.3285 Example 225 LCP/R-5/G-295 75.87 0.0131 Example 226 LCP/R-5/G-245 72.45 0.0235 Example 227 LCP/R-5/G-195 67.7 0.0347 Example 228 LCP/R-5/G-145 61.24 0.0439 Example 229 LCP/R-5/G-95 51.17 0.1652 Example 230 LCP/R-7.5/G-267.5 72.99 0.0168 Example 231 LCP/R-7.5/G-242.5 68.37 0.0279 Example 232 LCP/R-7.5/G-192.5 61.89 0.0363 Example 233 LCP/R-7.5/G-142.5 51.9 0.0889 Example 234 LCP/R-7.5/G-92.5 Example 235 LCP/R-10/G-140 62.56 0.0299 Example 236 LCP/R-10/G-90 52.66 0.0459 Example 237 LCP/R-12.5/G-87.5 53.28 0.0357 Example 238 LCP/R-5/G-0 5 3.76 Example 239 LCP/R-7.5/G-0 7.5 1.77 Example 240 LCP/R-10/G-0 10 0.61 Example 241 LCP/R-12.5/G-0 12.5 0.35 LCP Standard graphite CF-300 Melt-application compression molding Example 242 LCP/R-5/G-345 78.65 0.0041 Example 243 LCP/R-5/G-320 77.38 0.0046 Example 244 LCP/R-5/G-295 75.87 0.0049 Example 245 LCP/R-5/G-245 72.45 0.0055 Example 246 LCP/R-7.5/G-267.5 74.48 0.0051 Example 247 LCP/R-7.5/G-242.5 72.99 0.0055 Example 248 LCP/R-7.5/G-192.5 68.37 0.0064

TABLE 10 φM/% by Resistivity Constitution weight RD/Ω cm PPS, Standard graphite CF-300 Compression molding Example 249 PPS/R-15/G-85 53.93 0.0067 Example 250 PPS/R-10/G-190 68.86 0.0055 Example 251 PPS/R-10/G-140 62.56 0.0053 Example 252 PPS/R-10/G-90 52.66 0.0112 Example 253 PPS/R-5/G-295 75.87 0.0065 Example 254 PPS/R-5/G-245 72.45 0.0074 Example 255 PPS/R-5/G-195 67.7 0.0088 Example 256 PPS/R-5/G-145 61.24 0.0106 Example 257 PPS/R-5/G-95 51.17 0.0196 Comparative Example 258 PPS/R-0/G-300 75 0.0198 Comparative Example 259 PPS/R-0/G-250 71.4 0.0193 Comparative Example 260 PPS/R-0/G-200 66.6 0.0332 Comparative Example 261 PPS/R-0/G-150 60 0.0372 Comparative Example 262 PPS/R-0/G-100 50 0.0685 Comparative Example 263 PPS/R-5/G-0 5 5.02 Comparative Example 264 PPS/R-10/G-0 10 0.29 Comparative Example 265 PPS/R-15/G-0 15 0.093 PPS, Standard graphite CF-300 Injection molding Example 266 PPS/R-15/G-85 53.93 0.0149 Example 267 PPS/R-10/G-190 68.86 0.0159 Example 268 PPS/R-10/G-140 62.56 0.0214 Example 269 PPS/R-10/G-90 52.66 0.0347 Comparative Example 270 PPS/R-5/G-295 75.87 Comparative Example 271 PPS/R-5/G-245 72.45 0.0246 Comparative Example 272 PPS/R-5/G-195 67.7 0.0266 Comparative Example 273 PPS/R-5/G-145 61.24 0.0449 Comparative Example 274 PPS/R-5/G-95 51.17 0.1099 Comparative Example 275 PPS/R-0/G-300 75 0.0331 Comparative Example 276 PPS/R-0/G-250 71.4 0.0419 Comparative Example 277 PPS/R-0/G-200 66.6 0.0848 Comparative Example 278 PPS/R-0/G-150 60 0.24 Comparative Example 279 PPS/R-0/G-100 50 1.34 Comparative Example 280 PPS/R-5/G-0 5 1.58 Comparative Example 281 PPS/R-10/G-0 10 0.40 

1. A bipolar plate, end plate, or part of an end plate of a fuel cell, comprising a liquid-crystalline plastic and a conductive molding composition based on liquid-crystalline plastic, wherein the conductive molding composition comprises, as conductive constituents, A) carbon black and graphite, or B) carbon black and metal powder, or C) carbon black and graphite and metal powder, the carbon black has a specific surface area of from 500 to 1500 m²/g, and a dibutyl phthalate value of from 470 to 700 ml/l 100 g, and the graphite has a specific surface area of from 1 to 35 m²/g and a median particle size of from 50 to 450 βm.
 2. A process for producing a bipolar plate, endplate, or part of an endplate for fuel cells from a liquid-crystalline plastic molding composition wherein the plastic molding composition comprises, as conductive constituents, A) carbon black and graphite, or B) carbon black and metal powder, or C) carbon black and graphite and metal powder, the carbon black has a specific surface area of from 500 to 1500 m²/g, and a dibutyl phthalate value of from 470 to 700 ml/100 g, and the graphite has a specific surface area of from 1 to 35 m²/g and a median particle size of from 50 to 450 βm, which comprises production via compression molding.
 3. The process as claimed in claim 2, wherein the plastics molding composition has been pre-ground and has particle sizes of from 1500 to 50 βm.
 4. The process as claimed in claim 2, wherein the plastics molding composition has been pre-ground and has particle sizes of from 1,000 to 100 μm.
 5. The process as claimed in claim 2, wherein the plastics molding composition has been pre-ground and has particle sizes of from 800 to 150 μm.
 6. The molding as claimed in claim 5, wherein the molding is a bipolar plate, an end plate or a part of an end plate of a fuel cell.
 7. A process for producing a bipolar plate, endplate, or part of an endplate for fuel cells from a liquid-crystalline plastic molding composition wherein the plastic molding composition comprises, as conductive constituents, A) carbon black and graphite, or B) carbon black and metal powder, or C) carbon black and graphite and metal powder, the carbon black has a specific surface area of from 500 to 1500 m²/g, and a dibutyl phthalate value of from 470 to 700 ml/100 g, and the graphite has a specific surface area of from 1 to 35 m²/g and a median particle size of from 50 to 450 βm, which comprises production in a single step of a process, by bringing together the compounding step and the preparation step to give a single-stage process.
 8. The process as claimed in claim 7, wherein the method of production is injection-molding compounding with injection-compression molding unit.
 9. The process as claimed in claim 7, wherein the method of production is melt-application compression molding. 